Catalysis is the basis for many industrial/commercial processes in the world today. The most important aspect of a catalyst is that it can increase the productivity, efficiency and profitability of the overall process by enhancing the rate, activity and/or selectivity of a given reaction. Many industrial/commercial processes involve reactions that are simply too slow and/or efficient to be economical without a catalyst present. For example, the process of converting natural gas or methane to liquid hydrocarbons (a particularly desirable process for making liquid fuels) necessarily involves several catalytic reactions.
The conversion of methane or natural gas to hydrocarbons is typically carried out in two steps. In the first step, a methane-containing gas is converted into a mixture of carbon monoxide and hydrogen (i.e., “synthesis gas” or “syngas”). In a second step, the syngas intermediate is catalytically converted to higher hydrocarbon products, i.e., C5+, by processes such as the Fischer-Tropsch Synthesis.
Current industrial use of methane or natural gas as a chemical feedstock proceeds by the initial conversion of the feedstock to carbon monoxide and hydrogen by for example, steam reforming (the most widespread process), dry reforming, autothermal reforming, gas heated reforming, partial oxidation, or catalytic partial oxidation. Examples of these processes are disclosed in GUNARDSON, HAROLD, Industrial Gases in Petrochemical Processing 41–80 (1998). An example of catalytic partial oxidation is shown in U.S. Published patent application No. 20020013227 to Dindi et al., both incorporated herein by reference for all purposes. Steam reforming, dry reforming, and catalytic partial oxidation proceed according to the following reactions with methane as hydrocarbon gas feedtock, respectively:CH4+H2OCO+3H2  (1)CH4+CO22CO+2H2  (2)CH4+1/2O2→CO+2H2  (3)
While currently limited as an industrial process, catalytic partial oxidation (CPOX) has recently attracted much attention due to significant inherent advantages, such as the fact that heat is released during the process, in contrast to the endothermic steam and dry reforming processes.
The catalytic partial oxidation (“CPOX”) of hydrocarbons, e.g., methane or natural gas, to syngas has also been described in the literature. In catalytic partial oxidation, natural gas or methane is mixed with air, oxygen-enriched air, oxygen blended with a diluent gas such as nitrogen, or substantially pure oxygen, and introduced to a catalyst at elevated gas hourly space velocity, temperature and pressure to generate a syngas product, wherein syngas comprises a mixture of hydrogen (H2) and carbon monoxide (CO). As shown in Equation (3), the partial oxidation of methane yields a syngas product with a H2:CO molar ratio close to 2:1, which is more useful for the downstream conversion of syngas to chemicals such as methanol or to hydrocarbons such as liquid fuels, waxes, and/or lubricating oils, than is the 3:1 H2:CO ratio from steam reforming. However, both reactions continue to be the focus of research in the world today.
As stated above, these reactions are catalytic reactions and the literature is replete with varying catalyst compositions. The more preferred catalytic metals are Group VIII or noble metals, such as rhodium, platinum, or palladium. These metals are sometimes combined with secondary metals (also called promoter metals) to enhance their activity. Further, the catalyst compositions may include a support material such as alumina, silica, titania, zirconia, and the like.
Partial oxidation catalysts are generally prepared by uniformly dispersing one or more catalytic metal throughout a catalyst support matrix. One problem with this type preparation is that some preferred catalytic metals, e.g., Rh, Pt, Pd, are typically expensive. Regardless of how expensive or how high the loading, uniform dispersion inherently exiles a substantial amount of catalytic metal to the deep internal surfaces or “core” of a support. For purposes of this disclosure “deep internal surfaces” or “core” is intended to mean the region of the catalyst structure that is deeper than the smaller of (1) 300 μm as measured from the catalyst exterior surface or (2) the outer 30% of the catalyst volume as measured from the catalyst exterior surface directly inwards toward the center of the structure. It should be appreciated that volume here and further herein is contemplated to include both the solid and pore volumes that make up a particle structure. The catalyst metal exiled in this manner creates at least two disadvantages. First, catalytic metal inside the core region of the catalyst support structure will be underutilized. This is especially true in high space velocity reactors where only a relatively small fraction of the reactant gases are able to reach the core region. A second disadvantage is that the any products formed inside the core region will have to diffuse out of the core and the outer layer surrounding the core. The time it takes for the products formed inside the core region to leave the catalyst particle is significantly greater relative to the products formed at or near the exterior surface of the catalyst. More specifically, the difference is equal to the added time it takes for the products to diffuse out of the core. Thus, a further disadvantage is that the possibility for undesired side reactions becomes greater because the products and the reactants are exposed to active catalyst material for a longer period of time.
Applicants theorize, without wishing to be bound by the theory, that in processes having very high reactant space velocities, such as, for example, the catalytic partial oxidation of methane, the inner part of the catalyst particles (e.g., core) do not significantly contribute to the overall reaction productivity. In fact, Applicants postulate that the undesired secondary reactions are predominant in the inner region. Therefore, the chemical activity in the core of the catalyst particle will have the undesirable effects set forth above such as affecting the H2:CO molar ratio, lowering the H2 and CO selectivities, and increasing the reaction temperature. Additionally, the catalytic metal inside the inner core of the catalyst is underutilized with respect to the desired reactions. Therefore, the catalyst metal dispersed in the inner core of the particles contributes significantly to the cost without contributing significantly to the productivity, and indeed potentially decreasing the productivity, of the reactor.
Non-porous material (defined as a material with a surface area of less than 1 m2/g) would typically provide a catalyst core without significant diffusion of reactants/products and thus would overcome the production of undesirable by-products. However, the use of a non-porous support material for the catalyst core is not an acceptable solution, because non-porous materials do not have a sufficient amount of surface area on which to disperse the catalytic material. In addition, lack of sufficient surface area can prevent the reactions from igniting.
Thus, there is a desire to have a syngas production process, which minimizes the amount of expensive catalytic material in the core of catalyst particles and also limits access of the core to the reactants, thus limiting the extent of the occurrence of the undesirable secondary reactions and minimizing the effects of those undesirable reactions. Accordingly, research has focused on developing new catalysts that can reduce or eliminate the problems associated with the prior art catalysts. The approach taken is to more effectively using smaller amounts of catalytic metal or using the catalytic metals more efficiently while maintaining the required minimum amount of surface area. The present invention has been developed with these considerations in mind and is believed to be an improvement over the catalyst systems in the prior art.